Appointments to empty duchies, or duchies with, say, a single mountain region, could easily be used to remove lords from critical positions, like Margrave of the capital, or the realm's critical food producing rural, etc.
if the guy really wants the new duchy/lordship, then just have him step down from his old one. What's so hard about that? Send him a message: "Hey, you want the duchy of Keplerstan? If so, step down from lord of Eviltown so I can appoint you."